The present invention relates generally to the field of refrigeration systems and, more particularly, to a refrigeration system having a variable refrigerant charge.
Refrigeration systems generally include a compressor, condenser, expansion device and evaporator that are interconnected by a refrigerant fluid path. The compressor draws low pressure refrigerant gas through a suction line, compresses the low pressure refrigerant gas and then discharges it as a high pressure refrigerant gas into the high pressure side of the system. The high pressure refrigerant gas is cooled and liquified in the condenser and is discharged from the condenser as a high pressure liquid refrigerant. At the expansion device, the high pressure liquid refrigerant expands into a low pressure liquid refrigerant before entering the evaporator. The evaporator cools the refrigerated space of, for example, a freezer, by means of a cold wall of the freezer""s cabinet. Alternatively, room air may be directed across the evaporator coil by fans to cool the air before it enters the intended refrigerated space. The evaporator evaporates the low pressure liquid refrigerant and discharges a low pressure gaseous refrigerant into the suction line of the compressor and the cycle repeats.
The charge of refrigerant circulating within the refrigeration system is determined by several factors, including the desired cooling capacity of the system and the operating constraints of the refrigeration or heat exchange components of the system, particularly the compressor. For many applications, the desired charge of refrigerant is well within the operating capability of the compressor through the range of temperatures expected to be encountered so that the compressor does not place an operating constraint on the charge of refrigerant that can be used in the system to provide the desired cooling capacity.
However, for some refrigeration systems, such as ultra low temperature freezers that must be cooled from ambient to an operating point in the range of about xe2x88x9250xc2x0 C. to about xe2x88x9290xc2x0 C., the compressor places an operating constraint on the charge of refrigerant that may be used at elevated temperatures, so that the cooling capacity of the freezer at its operating point must be sacrificed. While additional charge of refrigerant may be desired for providing additional cooling capacity to the freezer at its operating point, the compressor is not capable of handing the additional refrigerant in the fluid path at elevated temperatures of the system without risking damage to the compressor.
Therefore, there is a need for a refrigeration system that safely and reliably operates to provide additional cooling capacity to the system at its operating point without risking damage to the refrigeration or heat exchange components of the system as it cools down from ambient to its operating point.
The present invention overcomes the foregoing and other shortcomings and drawbacks of refrigeration systems heretofore known.
While the invention will be described in connection with certain embodiments, it will be understood that the invention is not limited to these embodiments. On the contrary, the invention includes all alternatives, modifications and equivalents as may be included within the spirit and scope of the present invention.
A refrigeration system in accordance with the principles of the present invention includes a compressor, condenser, expansion device and evaporator that are interconnected by a refrigerant fluid path. A refrigerant expansion tank is provided in the system for varying the charge of refrigerant in the fluid path. Varying the charge of refrigerant in the system is particularly advantageous when additional cooling capacity to the system is desired at its operating point, but the operating constraints of the refrigeration or heat exchange components of the system, particularly the compressor, prevent the presence of additional refrigerant in the fluid path of the system while it operates at elevated temperatures before cooling down to its operating point.
In accordance with the principles of the present invention, the expansion tank adds additional refrigerant to the system only after it has reached, is near, or at least is approaching, its operating point. In this way, the operating constraints of the compressor that occur at elevated temperatures of the system are no longer present, and the compressor is therefore capable of handling the additional refrigerant at the lower temperatures of the system to provide the additional cooling capacity. The additional refrigerant is removed from the fluid path and contained within the expansion tank when the refrigeration system is turned off, and is ready to be added again to the system to provide the additional cooling capacity.
The expansion tank is connected to a suction line of the compressor through a valve and an expansion device, such as a capillary tube expansion device. The valve is a solenoid-controlled valve and includes a normally-open solenoid for controlling open and closed positions of the valve. When the valve is open, a predetermined amount of refrigerant stored in the expansion tank passes through the capillary tube expansion device to the suction side of the compressor. The capillary tube expansion device meters the flow of refrigerant from the expansion tank to the compressor to prevent damage to the compressor. The additional refrigerant provided by the expansion tank increases the charge of refrigerant in the refrigerant fluid path of the system to thereby provide additional cooling capacity to the system. The valve also permits the refrigerant to pass from the suction line back to the expansion tank so that when the valve is closed, a predetermined amount of the refrigerant is stored in the expansion tank and ready to be added again to the suction line of the system.
In accordance with one aspect of the present invention, the solenoid of the valve is controlled by a time-delay relay. When the refrigeration system is turned on, the contacts of the time-delay relay close to cause the solenoid-controlled valve to close. At this point, refrigerant contained within the expansion tank is stored until the valve is opened again.
After power has been applied to the refrigeration system, the system begins to cool down to its operating point. The relay contacts of the time-delay relay remain closed for a predetermined duration of time, such as eight (8) hours for example, to allow sufficient time for the system to reach its operating point. The solenoid-controlled valve remains closed as long as the relay contacts of the time-delay relay remain closed. When the predetermined duration of time expires, the time-delay relay opens its relay contacts which causes the solenoid-controlled valve to open. At this point, the refrigeration system is operating preferably at or near, or at least approaching, its operating point and refrigerant stored in the expansion tank is now permitted to pass from the expansion tank, through the valve and capillary tube expansion device, to the suction line of the compressor to thereby provide additional cooling capacity to the system.
When the refrigeration system is turned off, the relay contacts of time-delay relay remain open, and the solenoid-controlled valve remains open as well. In this way, refrigerant is permitted to pass from the suction line of the compressor back to the expansion tank so that when the valve is closed, a predetermined amount of the refrigerant is stored in the expansion tank and ready to be added again to the suction line of the system. The valve closes when the refrigeration system is turned on again which causes the relay contacts of the time-delay relay to close for the predetermined duration of time and the cycle repeats.
In accordance with another aspect of the present invention, the solenoid of valve is controlled by a relay that is operatively coupled to a relay control and a temperature probe. In this embodiment, the temperature probe is preferably mounted in a cabinet of the refrigeration system or other intended refrigerated space of the system.
When the refrigeration system is turned on, the contacts of the relay close to cause the solenoid-controlled valve to close. At this point, refrigerant contained within the expansion tank is stored until the valve is opened again.
After power has been applied to the refrigeration system, the system begins to cool down to its operating point. During this time, the relay control, which may comprise a microprocessor or other controller coupled to a relay driver by way of example, continuously or intermittently monitors the temperature sensed by the temperature probe. The relay contacts of the relay remain closed until the relay control determines that the cabinet or other portion of the refrigeration system has reached a predetermined temperature as sensed by the temperature probe. The solenoid-controlled valve remains closed as long as the relay contacts of the relay remain closed.
When the relay control determines that the cabinet or other portion of the refrigeration system has reached a predetermined temperature as sensed by the temperature probe, the relay control causes the relay to open its relay contacts which causes the solenoid-controlled valve to open. At this point, the refrigeration system is operating preferably at or near, or at least approaching, its operating point and refrigerant stored in the expansion tank is now permitted to pass from the expansion tank, through the valve and capillary tube expansion device, to the suction line of the compressor to thereby provide additional cooling capacity to the system.
When the refrigeration system is turned off, the relay contacts of relay remain open, and the solenoid-controlled valve remains open as well. In this way, refrigerant is permitted to pass from the suction line of the compressor back to the expansion tank so that when the valve is closed, a predetermined amount of the refrigerant is stored in the expansion tank and ready to be added again to the suction line of system. The valve closes when the refrigeration system is turned on again which causes the relay contacts of the relay to close until the predetermined temperature condition is met again and the cycle repeats.
The refrigerant expansion tank of the present invention varies the charge of refrigerant in the system to provide additional cooling capacity to the system when it has reached, is near, or at least is approaching, its operating point. In this way, the operating constraints of the compressor that occur at elevated temperatures of the system are no longer present, and the compressor is therefore capable of handling the additional refrigerant at the lower temperatures of the system to provide the additional cooling capacity. The additional refrigerant is removed from the fluid path and contained within the expansion tank when the refrigeration system is turned off, and is ready to be added again to the suction lines of the system to provide the additional cooling capacity in accordance with the principles of the present invention.